1. Field of the Invention
The present invention relates to an automatic thin-film measuring apparatus, and more particularly to an automatic thin-film measuring apparatus which measures the difference in weight of a semiconductor substrate before and after a thin film is formed thereon, and finds the thickness of the thin film.
2. Description of the Related Art
Typically, the thickness of a thin film formed on a semiconductor substrate is found based on the weight of the thin film. In this method, assuming that the thin film is uniform in thickness on the semiconductor substrate, the weights of the semiconductor substrate before and after the thin film is formed are measured, and the weight of the thin film is found based on the difference between the weights. Subsequently, the thickness of the thin film is found by dividing the weight of the thin film by the surface area of the semiconductor area (the area where the thin film is formed) and the specific gravity of the thin film.
A conventional thin-film measuring apparatus using such a method is illustrated in FIG. 9. In this thin-film measuring apparatus, the thickness of a thin film is measured by a measurement method shown in FIG. 10.
Referring to FIG. 9, a semiconductor substrate 6 is set on a pan 3 in a manual precision balance 11 by which the weight of the semiconductor substrate 6 is measured. In the precision balance 11, weight measurement is started in response to the actuation of a measurement start switch 12, and a measured value is displayed on a display portion 13 and transmitted to a data processing unit 7.
The conventional thin-film measuring apparatus has the above construction, and the measurement procedures thereof are conducted as follows. First, the semiconductor substrate 6, on which a thin film has not yet been formed, is laid on the pan 3 of the precision balance 1 with tweezers or the like, and an unillustrated door is shut by the operator. Then, the measurement start switch 12 is pressed when a measured weight value becomes stable, weight measurement is performed (Step S38). The measured weight value is transferred to the data processing unit 7 and stored as data on pre-formation measurement (weight measurement before a thin film is formed) (Step S39). Then, the semiconductor substrate 6 is taken out of the precision balance 11 with tweezers or the like.
Subsequently, the semiconductor substrate 6 on which a thin film has been formed is set on the pan 3 of the precision balance 11 with tweezers or the like, and the door is shut. The pre-formation measurement has been conducted on the semiconductor substrate 6. When a measured weight value becomes stable, the measurement start switch 12 is pressed, thereby starting weight measurement (Step S40). The measured weight value is transferred to the data processing unit 7 and stored as data on post-formation measurement (weight measurement after the thin film formed) (Step S41). The semiconductor substrate 6 is then taken out of the precision balance 11 with tweezers or the like. The weight of the thin film is found by subtracting the measured weight value in the pre-formation measurement from the measured weight value in the post-formation measurement in the data processing unit (Step S42), and the thickness of the thin film formed on the semiconductor substrate 6 is obtained by dividing the weight by the surface area of the semiconductor substrate 6 and the specific gravity of the thin film (Step S43).
In the thin-film measuring apparatus described above, the weight measurement of the semiconductor substrate 6 requires the use of human hands, the indicated value (for example, expressed in a digital number in seven figures) of the precision balance 11 may be changed due to vibrations and so on applied to the precision balance 11 when the semiconductor substrate 6 is set on the pan 3 or when the door is shut, and the measurement start switch 12 is turned on and the indicated value is read before the indicated value is stabilized. The measured value varies depending upon the timing of actuation of the measurement start switch 12 by the operator. For example, the measurement change rate in the use of the conventional precision balance 11 is approximately .+-.5% when a semiconductor substrate of approximately 25 g in weight (6-inch substrate) is used in the above thin-film measuring apparatus.
If the weight measurement is performed in a state where the semiconductor substrate 6 is not cooled to the room temperature, for example, at approximately 60.degree. C., convection arises in the precision balance 11 and reduces measurement precision. Furthermore, the measurement precision is reduced by low-frequency disturbances resulting from the building and other apparatus. In addition, since the conventional thin-film measuring apparatus cannot be incorporated into a FA (factory automation) system, it is impossible to automate the measurement.